Apex domains cannot have CNAME records.
Question and response
Question
dig fu.tuke.sk. NS
Answer
fu.tuke.sk. 5 NS a.ns.tuke.sk.
fu.tuke.sk. 5 NS c.ns.tuke.sk.
fu.tuke.sk. 5 NS b.ns.tuke.sk.
fu.tuke.sk. 5 NS zub.fei.tuke.sk.
Authority
The response has no authority records.
Additional
a.ns.tuke.sk. 5 A 147.232.3.3
b.ns.tuke.sk. 5 A 147.232.16.3
c.ns.tuke.sk. 5 A 193.87.100.3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0